ci: install beautifulsoup4 so danawa price search tests can import bs4

The new scripts/test_danawa_price_search.py imports danawa_search.py,
which requires beautifulsoup4. CI only runs npm ci, so the bs4 import
fails with 'beautifulsoup4 is required: python -m pip install
beautifulsoup4' and the validate job exits with code 1.

Install beautifulsoup4 via pip before running npm run ci so the
Python test suite can import danawa_search and run the new payment
badge regression tests.

.github/workflows/ci.yml

@@ -17,5 +17,8 @@ jobs:
           node-version: 20
           cache: npm
 
+      - name: Install Python test dependencies
+        run: python3 -m pip install --user beautifulsoup4
+
       - run: npm ci
       - run: npm run ci